Misfire woes

'06 Liberty 3,7. Misfire on cyl 4. Scoped for secondary waveform on its COP and got no signal (tested adjacent COPs and got a good pattern). Swapped #4 COP with one of those that tested good and, again, no secondary waveform. I then disconnected COP primary plug (which is 2 pin) and probed the red wire and got momentary battery voltage with key-on and constant battery voltage with engine running. Just for laughs, I plugged COP #4 power plug into cyl #2 COP next to it and tested for secondary again. Still nothing. Did not test injector on 4.

Is it safe to assume I have a problem on #4's signal wire, Either an open or a problem with driver inside PCM? I don't know if the PCM could have possibly detected a problem with COP 4 and disabled the trigger.

It’s possible something smoked on the PCM board. I’ve seen a post around here that mentioned finding something similar.

Ksat said:

'06 Liberty 3,7. Misfire on cyl 4. Scoped for secondary waveform on its COP and got no signal (tested adjacent COPs and got a good pattern). Swapped #4 COP with one of those that tested good and, again, no secondary waveform. I then disconnected COP primary plug (which is 2 pin) and probed the red wire and got momentary battery voltage with key-on and constant battery voltage with engine running. Just for laughs, I plugged COP #4 power plug into cyl #2 COP next to it and tested for secondary again. Still nothing. Did not test injector on 4.

Is it safe to assume I have a problem on #4's signal wire, Either an open or a problem with driver inside PCM? I don't know if the PCM could have possibly detected a problem with COP 4 and disabled the trigger.

Click to expand...

I posted a detailed explanation of how to build/install (a) Fuel Injector and (b) Ignition Coil wiring harnesses from scratch. If you do this, do not use silicon wire... lesson learned was that the wire strands in silicon are very flexible... so use 18awg TXL stranded wire for the injector wires and 14awg TXL for the coil wires.

To clarify, did you physically swap your COPs or just rearrange them while they were still attached to their wires?

The true test is to unplug, swap COP positions, plug in the correct wire per cyl, start car, drive it and see if the problem stays with the COP instead of the cyl. If it stays with the COP, bad COP. If it stays with the cyl, then you have the same problem I have.

If your coils are ok (problem is cyl specific), look up "how to make a noid light" and attach that to your injector connector to do a simple bad-injector test as follows:
1. 2 pieces of 14awg solid wire about 2' long each (length so you can position the bulb where you can see it from the drivers seat)
2. 12v led bulb with pins (I had some bulbs and wire laying around, so didn't cost me anything)


3. Attach 1 wire to each pin (I used alligator clips crimped onto the ends of the bared wires)
4. Stick the other ends into each of the exposed injector connector plug coming from the PCM and power
5. Start you car and watch the light. If it pulses regularly, at least the circuit is working... but that doesn't necessarily mean you are getting enough power all the way to the injector.

Try that and post your results back.

Firstly, thank you all for the replies.

To answer some questions, To clarify, I could feel the engine missing a minute or 2 after start up yesterday. I scanned it and got a misfire code for #4. I went to #4 COP and tested for presence of secondary waveform with my scope with engine running and got none. To make sure my scope was okay, I checked 2 and 6's waveforms and got good patterns on those.

I next removed 4's COP and swapped it with #2, which tested good above. Restarted engine and still no waveform on 4. I tested 2 and got a signal, so that should confirm there wasn't a problem with 4's original COP.

I tested for power on the primary for 4 (with it unplugged) with my meter and got ~+12V which rose to 14ish with engine running.

Thank you for the instructions on constructing a noid light. Because I have a scope, I think I'll be able to test for presence (or lack) of a trigger on 4 with that. It's been cold & windy today, so haven't been brave enough to go out and try to do that.

Some info that may be related to this problem is I've been dealing with some failing lifters mainly on bank 2 where #4 is and have been getting a number of misfire counts when viewed on mode 6. Someone on another forum suggested the PCM may have disabled trigger on 4 in response to these misfires occurring.

Home made noid looks like this. Handy because you can simply plug the electrodes into the FI injector connector housing and tape the bulb to the top of the air intake where you can see it from the drivers seat when you start the KJ.

I just went through the same song and dance with the cylinder 4 misfire issues. After chasing the problem in a dozen different ways I realized the PCM was the issue.

I installed the new PCM and like magic the misfire was gone. The circuit for cylinder 4 going bad in the PCM is apparently a common issue.

That is great news. If the code pops again, please let me know.

I replaced my PCM and it had no long term effect. When you replace the PCM you disconnect the battery, which effectively causes a hard reset of the PCM. My P0304 came back after about 120 miles. Subsequent hard resets might go as far as 200 miles before the code popped again, but 120 miles seemed consistent.

If this happens do not be discouraged. Just update this thread and we can do some additional q&a.

What finally seems to have worked for me was having all 6 of my fuel injectors replaced with Bosch OEMs. It seems I had 5 cheap chinese POSs and 1 Mopar OEM, and I suspect that the fuel volume of the POS parts was completely wacked. I've gone >300 miles with no misfire and the engine is smooth as glass.

I will post a complete run down of everything I've done since the engine swap in a week or two. I still have a slightly rich bank 2 long term fuel trim, and I think replacement of the ignition coils will solve that last problem. Once that's done and tested, everything will be shared in a separate posting.

You were right on the money, reload40s. Bad PCM. Specifically, a bad coil driver, I imagine. I scoped the ignition primary on 4 and got no ground control signal. Traced the wire back to PCM, tested from there and still got nothing. I've been hitting ebay for a "plug and play" programmed one, which hopefully will work w/o additional requirements.

I've been driving the thing on 5 cylinders right now and, to my dismay, it's been running terrible. Bucking like crazy, no power and can barely make it up the hills. I unplugged the upstream O2 on bank 2 thinking perhaps the air being pumped into the exhaust stream by 4 was skewing the sensor's readings, but it hasn't helped.

Please clarify, are you still throwing a misfire code?

The moment a misfire code is thrown, the PCM will kill the signal to the problem cylinder until you reset the PCM. The cylinder will not fire until you either (a) clear the code with a scanner reset which is good for about 50 miles, or (b) disconnect both battery cables and touch the end connectors together *BEWARE DO NOT TOUCH THE POSTS, TOUCH THE LOOSE CABLES*. After a reset all cylinders will work again unless you have a fried injector or/and coil.

If you are still misfiring, it may be a simple case of a broken fuel injector or ignition coil connector. Did you inspect the housings on those 2 connectors? Are the locking tabs still functional (wiggling will break the connection and throw a misfire code)? Are the waterproof seals still in place (trapped road water will short the circuit and throw a code)?

Even if they look ok but there is kinking/abrasion/crumbly isulation and you are not positive, replacing the connectors is a cheap checklist item.

If you decide to do this, make sure the new pigtail is attached such that each pigtail wire cavity attaches to the correct circuit wire (IOW trace the connector cavity to the correct color-coded wire according to your Jeep Repair Manual, I have a 2003 and the 2006 has a different PCM and probably different wire color-coding, so you have to look this up on your own).

Go here if you need to splice on a new connector select the correct pigtail here: https://www.standardbrand.com/en/products/electrical/connectors/electrical-connectors

Use a good crimper (the best: https://www.amazon.com/Crimping-Tool-Heat-Shrink-Connectors/dp/B07GFXHH91?ref_=ast_sto_dp&th=1 ), and use heatshrink butt connectors ( https://www.amazon.com/stores/page/...d5e5-5efd-44a1-b0e4-91ff33cc4f9c&ref_=ast_bln )

Note that PCMs are programmed very specifically to your vehicle build sheet, and the best way to make sure you get a matching PCM requires your VIN.

If you opt to replace your PCM, strongly recommend getting a new one from Flagship 1 (highly reliable).

Thanks for the detailed reply, dtennes. Yes, I'm still throwing a misfire code, which is expected since the cylinder is still misfiring. I'll give your idea a shot and clear the code/touch the cables together and see what happens, though I don't have much hope it will work.

The connections on both the ignition coil and injector are good. When scoping the coil, I got battery voltage on the ground control side of it, which proves the coil is receiving the power fed to it. When scoping the injector, I got a good waveform pattern for about 5 seconds after startup, then it goes flat. That is expected, as the PCM monitors cylinders for misfires and cuts fuel to prevent catalyst damage when it detects the condition.

Note that PCMs are programmed very specifically to your vehicle build sheet, and the best way to make sure you get a matching PCM requires your VIN

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Yea, I've noticed most selling programmed PCMs are asking for the VIN before they'll ship them out. Thanks for the recommend on Flagship.

Car Computer Exchange is another good vendor for these. I found them through Lost Jeeps. They asked for a lot of details including the VIN, Build Date, Serial Number from the old PCM and more. They sent me a perfect plug and play replacement.

Thanks for the suggestion. I ended up going with one on eBay, just a little while ago. Only around $70, which includes programming- almost 3X less than what Flagship One wanted. We'll see how it goes.

After talking to several people about it, apparently the most important thing is to make sure the part number of the replacement PCM matches up with the one that's currently in there (excluding the last letter, as that just indicates the date of manufacture). For example, the bum one that's in my vehicle right now is part # P05094939AB, so any PCM that includes "P05094939A" will work. In order to complete programming, the VIN and current odometer reading have to be provided. The seller said they would clear the SKIM functionality.

Sounds like you got it!